Due to recent changes in the species, size and quality of standing timber available for harvest throughout the world, composites of lignocellulosic materials have replaced traditional solid sawn lumber for use in many structural applications. Many of these composites are used in applications, which require resistance to wood-destroying organisms such as fungi and various insects. Accordingly, this requires treatment with a wood preservative.
Traditionally, solid wood products are dipped or pressure treated with solutions of preservative chemicals. However, the nature of a composite material makes it possible to incorporate a preservative into the product during its manufacture. This decreases total production costs and yields a superior product in which the composite has a constant loading of preservative throughout its thickness.
Borates have been used as broad-spectrum wood preservatives for over 50 years. Their benefits include efficacy against most wood destroying organisms such as fungi, termites and wood-boring beetles. Coupled with their low acute mammalian toxicity and low environmental impact, their fungicidal and insecticidal properties have resulted in them being considered the wood preservative of choice for most structural or construction applications. Borates such as boric acid, borax, disodium octaborate tetrahydrate (commercially available as Tim-bor® Industrial Wood Preservative from U.S. Borax Inc.) and, more recently, zinc borate are well accepted as wood preservatives. Generally, boric acid, borax and disodium octaborate are used for treating solid wood products by dip or pressure treatment. However, these preservatives are readily soluble in water and can be incompatible with many resin systems used in producing composite products, resulting in an adverse effect on the internal bond strength of the resultant composites and poor mechanical strength. Anhydrous borax and zinc borate have been used successfully at relatively low levels with some resin systems, such as the phenol-formaldehyde resins, to produce composites with acceptable internal bond strength. See Knudson et al., U.S. Pat. No. 4,879,083. Zinc borate is also particularly preferred because its inherent low solubility reduces leaching and loss of the active ingredients in laboratory tests and in high moisture hazard environments in service. Although the low solubility borates of Knudson et al., especially zinc borate, have been used successfully to treat wood composites such as oriented strand board (OSB), waferboard, fiberboard and particleboard, they suffer from several specific problems related to their low solubility. For example, while they perform very well in laboratory decay tests in high humidity situations, or after leaching, if the tests are carried out without leaching and at lower ambient relative humidities (20-30% moisture content in the wood) a limited amount of fungal decay can occur.
This invention provides composites made from wood and other lignocellulosic materials which are resistant to attack by wood destroying organisms such as fungi and insects, have excellent internal bonding strength, low inherent leaching and yet still provide both rapid and long-lasting performance against decay fungi and insects at low as well as high moisture content.